Bearings, crankshafts, and other analogous parts are important to support rotating units and sliding units of machinery. These parts are often used in severe environments because they receive a considerably high contact pressure (contact surface pressure) and may receive a varying external force. For this reason, steels to be used as materials for the parts require satisfactory durability.
Such requirement has become more and more exacting with higher and higher performance and smaller and smaller weights of machinery. To improve the durability of shaft or bearing parts, technical improvements in lubricity are important, but improvements in rolling-contact fatigue properties of steels are particularly important.
High-carbon-chromium bearing steels such as SUJ2 prescribed in Japanese Industrial Standard (JIS) G 4805 (1999) have been used as materials for bearings for use in automobiles, industrial machinery, and other various applications. The bearings, however, are disadvantageously susceptible to fatigue fracture caused by very fine defects (e.g., inclusions) because they are used in severe environments typically as inner and outer races and rolling elements of ball bearings and roller bearings where the contact pressure is very high. To solve this disadvantage, attempts have been made to improve bearing steels so as to prolong their rolling-contact fatigue lives themselves to thereby reduce the maintenance frequency.
For example, Patent Literature (PTL) 1 proposes a technique relating to a bearing steel. This technique specifies Ti and Al contents and performs a heating treatment after spheroidizing. This controls the amounts of fine particles of titanium carbide, titanium carbonitride, and aluminum nitride and thereby reduces the size of prior austenitic grains. Thus, the bearing steel may have better rolling-contact fatigue properties.
According to the technique, however, a very high titanium content of 0.26% or more is required, and this disadvantageously increases the steel cost and impairs the steel workability. The resulting steel manufactured by the technique suffers from the formation of coarse titanium nitride particles during casting and may have unevenness in fatigue life due to the formation of precipitates (titanium nitride particles). In addition, the steel has a high aluminum content of 0.11% or more and disadvantageously suffers from cracks and flaws caused by Al-containing nitrogen compounds formed during casting and rolling, thus resulting in poor manufacturability.